1. Field of the Invention
This invention relates to an optical recording medium, particularly to an optical recording medium improved in recording and reproduction characteristics and storage stability which can be used for optical disc or optical card.
2. Related Background Art
Generally speaking, optical recording medium such as optical disc and optical card can memory high density information by making optically detectable small (e.g. about 1.mu.) pits formed on a thin recording layer provided on a substrate in the form of a spiral or circular and linear track. For writing information on such a disc, laser converged on the surface of a laser sensitive layer is scanned and only the surface irradiated with the laser beam forms pits, which pits are formed in the form of spiral or circular or linear track. The laser sensitive layer can absorb the laser energy to form optically detectable pits. For example, according to the heat mode recording system, the laser sensitive layer can absorb heat energy and form small concavities (pits) through evaporation or melting at that site. On the other hand, according to another heat mode recording system, by absorption of the irradiated laser energy, a pit having optically detectable density difference at that site can be formed.
As the optical recording medium to be used in this kind of optical recording and reproduction device, there have been known those constituted of a substrate, a metal reflective layer and a recording layer containing a dye. Such optical recording medium is formed so as to detect easily the reflected light by permitting the laser beam incident on the recording layer containing a dye to reach the metal reflective layer to be reflected thereagainst, and the above metal reflective layer in this case is provided for supplementing the amount of reflected light which is insufficient only with the recording layer.
When a dye with great light absorption for laser beam is used as the organic dye thin film, light absorption reflective film exhibiting metallic lustre (reflectance 10 to 50%) is obtained to give an optical recording medium capable of laser recording and reflective reading. Particularly, when a semiconductor laser of oscillation wavelength of 700 to 850 nm is used as the laser light source, there is the advantage that the device can be made compact and low cost.
However, because the organic dye thin film is generally susceptible to material change caused by heat and light, there have been involved the problem that recording reproduction characteristic and storage stability are lowered.
Also, since optical recording medium performs recording with strong laser power and further reproduction with weaker laser power by use of the same laser as in the recording method as described above, the threshold value to laser power is required to be distinct. However, for organic dye thin films, distinct threshold value as mentioned above can be obtained with difficulty, as compared with inorganic thin films, and tends to be deteriorated during reproduction.
For prevention of such deterioration of organic dye thin film, there has been proposed an invention in which a photostabilizer such as aminium compound or diimonium compound is added to the organic dye thin film (GB 2155811A).